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I fully agree on the cypher then MAC policy. Deciphering is time consuming and by feeding bogus data could be used in a DOS attack. Without prior checking it may be possible to indirectly probe the deciphering process.

What I am not so sure is the choice of CTR. I currently prefer CFB.



... because?


CTR cyphers a value that changes with only a few bits, while CFB uses previous output which is much more random, though exposed, since it is the ciphered text.

PS: http://en.wikipedia.org/wiki/Block_cipher_modes_of_operation makes it clear.


Security under those circumstances is a design requirement for AES. CTR is secure if AES is secure. If known plaintext pairs differing in only a few bits break AES, we have bigger problems than CTR mode.




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